In the recent years, wavelength multiplexing optical communication systems have been investigated and developed very actively, and studies have taken place to develop a system which is capable of increasing the number of channels in signal light in answer to the demand for communications.
In addition, a desire exists to upgrade the amplification scale of an optical amplifier, being an essential component of this wavelength multiplexing optical communication system, in accordance with the increase in the number of channels. A similar need also exists on an optical amplifier included in a light wave network or the like.
To meet such a requirement, consideration may be given to a construction in which an optical amplifier capable of amplifying multi-wavelength (for example, approximately 32 channels) multiplexed signal light is introduced from the initial operation into an optical communication system, thus coping with the increase in the number of channels.
In this instance, although being equipped with an excitation light source, for handling the multi-wavelength multiplexed signal light, the optical amplifier is required to be provided with an excitation light source which is capable of supplying a large quantity of excitation light.
However, in addition to the fact that the excitation light source is usually expensive, a small number of channels (for example, approximately 4 channels) are frequently used at the beginning of the system operation, and therefore, if such an optical amplifier capable of dealing with multi-wavelength multiplexed signal light is employed from the first system operation, there is a problem in that the initial investment for the equipment increases to lower the investment efficiency.
For this reason, in order to enhance the equipment investment efficiency, consideration can be given to that another excitation light source is added to the existing optical amplifier in accordance with an increase in the number of channels in signal light while the optical communication system is in operation.
However, if the additional excitation light source is introduced into the control loop of the preexisting excitation light source, the control of these excitation light sources can frequently go unstable. More specifically, supposing that the quantity of an excitation light required for when the optical amplifier obtains a predetermined gain (excitation optical power) is P, difficulty is encountered to singly determine the combination of excitation optical powers of these two excitation light sources to be P in total, so that a plurality of stable operating points appear, which results in unstable excitation light source control.
On the other hand, consideration should also be given to the fact that, after an additional excitation light source is installed to increase the number of channels in signal light, the number of channels is decreased depending on the demand for communications while the optical communication system is in operation, and further, that a need can occur to remove the additionally installed excitation light source.
Accordingly, in the optical communication system being in operation, in order to coping with the increase/decrease in the number of channels, there is a need to provide a construction which allows the installation or the removal of an additional excitation light source without interfering with the working channels.
The present invention has been developed in consideration of such problems, and it is therefore an object of this invention to provide an optical amplifier, an excitation light source control method for use in an optical amplifier and an optical amplifier control method which are capable of stably installing or removing an additional excitation light source in accordance with an increase/decrease in the number of channels in signal light even if the optical communication system is in operation.